1. Field of the Invention
The present invention relates to a charger of a secondary battery, and more particularly to a charger that efficiently charges a secondary battery such as a mobile apparatus.
2. Description of Related Art
In recent years, with the development of high capacity batteries used in mobile apparatuses and the development of safety designs of the mobile apparatuses during use and charging, techniques for safely and efficiently charging secondary batteries are required. The mobile apparatus charging system typically employs a configuration of using an external power supply typified by an AC adapter as the input source, and charging the secondary battery via a charger having a series regulator configuration. The series regulator is provided with a control transistor between the input and output, and controls output voltage (the charging voltage of the secondary battery) and output current (the charging current of the secondary battery) using the control transistor as a variable impedance. Hereinafter, the control transistor will be referred to as the charging control transistor of the charger.
The external power supply is also typically provided with an overcurrent protection function. Standard overcurrent protection has a drooping characteristic whereby, when the output current (that is, the input current for the charger or the charging current to the secondary battery) of the external power supply reaches the limit value, the output voltage of the external power supply (hereinafter referred to as the supply voltage from the external power supply, as viewed from the charger) decreases with the current value maintained. For the charger, the external power supplies can be broadly divided into two types according to the drooping characteristic. One is an external power supply whose current limit value is less than the maximum charging current, and the other is an external power supply whose current limit value is greater than or equal to the maximum charging current.
In the initial charging stage when the secondary battery voltage is low and the charger attempts to charge the battery using the maximum charging current, if the current limit value of the external power supply is less than the maximum charging current of the charger, the supply voltage from the external power supply is drooping and the charging control transistor operates in a saturated state. On the other hand, when the current limit value of the external power supply is greater than or equal to the maximum charging current of the charger, the supply voltage from the external power supply does not decrease, and therefore the difference voltage (Vc−Vb) between supply voltage Vc from the external power supply and voltage Vb of the secondary battery is applied to the charging control transistor, and the loss Pd=(Vc−Vb)×Ichmax obtained by multiplying the difference voltage by the maximum charging current Ichmax occurs.
Patent Document 1 (Japanese Patent Application Laid-Open No. 1994-253467) discloses a charger that is provided with a switching step-down type DC/DC converter between a charging control transistor and an input terminal to which supply voltage is applied from an external power supply, and efficiently lowers the supply voltage from the external power supply using this DC/DC converter and decreases the loss caused by the charging control transistor.
FIG. 1 is a block diagram showing the configuration of the charger described in the above Patent Document 1.
In FIG. 1, 1 is an AC adapter that supplies DC voltage Vc, 2 is a secondary battery, and 10 is a charger. Charger 10 is configured with step-down type DC/DC converter 20 that converts at high efficiency supply voltage Vc to voltage Va, charging control section 30 that performs control of supplying charging current Ich to secondary battery 2 based on voltage Va from step-down type DC/DC converter 20, and voltage difference detection circuit 40 that detects the voltage difference between output voltage Va of DC/DC converter 20 and battery voltage Vb.
Step-down type DC/DC converter 20 is configured with control transistor 21, inductor 22, commutation diode 23, capacitor 24 and DC/DC control section 25, and charging control section 30 is a series regulator configured with charging control transistor 31, charging current detection resistor 32 and charging control circuit 33. The output of voltage difference detection circuit 40 is fed back to DC/DC converter 20, and DC/DC converter 20 controls output voltage Va so that the detected difference voltage (Va−Vb) becomes constant. In step-down type DC/DC converter 20, the difference voltage (Va−Vb) is kept constant, thereby preventing power loss upon supply of charging current Ich in charging control section 30.
FIG. 2 is an operation waveform diagram of each section of the above-mentioned charger 10, and shows the sectional time charts from the initial charging stage, of secondary battery 2 to charge completion. FIG. 2 shows supply voltage Vc from AC adapter 1, output voltage Va of DC/DC converter 20, battery voltage Vb of secondary battery 2 and charging current Ich from charging control section 30. As shown in FIG. 2, in the initial charging stage when battery voltage Vb is low, charging is carried out at a fixed maximum charging current (see CC charging in FIG. 2), and, when battery voltage Vb reaches a predetermined value near the full charge voltage, the charging switches to constant voltage charging (see CV charging in FIG. 2). During all periods, output voltage Va of DC/DC converter 20 changes in response to the change of battery voltage Vb. Thus, in the initial charging stage when battery voltage Vb is low, output voltage Va becomes a low value, and the power loss of charging control section 30 during CC charging decreases as expressed by Pd=(Va−Vb)×Ichmax. Furthermore, the power loss of the overall charger 10 requires consideration of the loss for DC/DC, which is expressed by the following equation 1. Thus, the total power loss of charger 10 is expressed by the following equation 2.Pd(DC/DC)=Va×Ichmax×(1−efficiency rate)/efficiency rate  (Equation 1)Pd=(Va−Vb)×Ichmax+Va×Ichmax×(1−efficiency rate)/efficiency rate  (Equation 2)
However, in such a charger of prior art, there is a problem that the operation of the DC/DC converter becomes unstable when the current limit value of the external power supply is less than the maximum charging current of the charger or when the battery voltage is low.
For example, charger 10 having the configuration of FIG. 1 is designed under the premise that the current limit value of the external power supply is greater than or equal to the maximum charging current Ichmax of charger 10. When the current limit value of the external power supply is less than the maximum charging current Ichmax of charger 10, supply voltage Vc from the external power supply, which is the input voltage of DC/DC converter 20, decreases during CC charging of the initial charging stage, and therefore the operation of DC/DC converter 20 becomes unstable. DC/DC converter 20 controls output voltage Va so that the detected difference voltage (Va−Vb) becomes constant, and therefore, when supply voltage Vc is extremely small, DC/DC converter 20 becomes saturated, and the operation becomes unstable. That is, although the operation is not performed as difference voltage output operation of DC/DC converter 20, power is supplied to DC/DC converter 20, and therefore the operation results in a meaningless waste of power and the amplification of noise. Thus, in terms of safety, it is not preferable that charging control section 30 performs charging when the operation of DC/DC converter 20 is unstable.
In addition, there is a risk of causing damage to secondary battery 2 if quick charging is performed at the maximum charging current when the battery voltage is low. For example, in the initial charging start stage indicated by CC charging in FIG. 2, if the chargeable charging current is supplied to secondary battery 2 to the fullest, secondary battery 2 is damaged, resulting in deterioration, and further, causing the risk of heat generation.